The callosal visual connections of the cat provide a model for studying the phenotypes of cortical axons and their differentiation. The terminal arbor of a callosal axon develops in several successive stages. At each stage, the arbor approximates the adult phenotype more closely. This is achieved through two mechanisms: (1) exuberant, but increasingly constrained, growth and (2) partial deletion of previously generated parts of the arbor. This differentiation is controlled by interactions of the axon with its cellular environment, and by visual experience. It might have played a permissive role in the evolution of the cerebral cortex by enabling adjustments of cortical connectivity to changes in the number, size, internal organization and cellular composition of cortical areas.